Blood flow probes based on the so-called Faraday principle are standard devices used to measure blood flow velocity through veins and arteries. In one form of these devices, they are in the form of lumens which may be placed about the vein or artery. The lumen contains electromagnetic coils and electrical terminals. The energizing of the coil generates a magnetic field and a potential is generated between suitably placed electrodes which is proportional to the volumetric rate of blood flow.
Reference may be had to Kolin U.S. Pat. No. 3,309,924; Westersten U.S. Pat. No. 3,316,762; and Hickman U.S. Pat. No. 3,580,071 for details of construction of such flow probes. See also "Single-Coil Coreless Electromagnetic Blood Flow Meters" by Kolin and Wisskaupt in IEEE Transaction on Bio-Medical Electronics, Vol. BME 10 No. 2, April 1963.
The prior art probes as illustrated by the above patents had a number of difficulties. In order to insert the artery or vein into the lumen, an opening is necessary. Two procedures for this purpose have been employed in the prior art. In one form (see Kolin U.S. Pat. No. 3,309,924), the lumen is split into two parts; and a hinge is provided at adjacent ends of the semicircular halves of the lumen, the other end being free. Another solution is to provide a window in the lumen through which the artery or vein may be passed and a shutter is used to close the window.
In the hinged form (see, for example, Kolin U.S. Pat. No. 3,309,924), there is a problem of holding the free ends of the lumen section in locked position to be sure they do not separate. The angle which may be subtended by the coils is limited by the space occupied by the hinge.
Another problem arises from the hazards of injury to the blood vessel by the closing of the hinged section about the blood vessel. Another hazard resides in the presence of the hinge which may cause injury or irritation to the surrounding tissues.
This type of lumen has been replaced by lumens in which a window is provided which may be closed by a plug as referred to above. This type has been used in two forms. In the above Kolin type, a single coil has been employed. The angle subtended by the coil limits the size of the opening. In order to provide a sufficient arcuate angle subtended by the coil and also pass the blood vessel, the internal diameter of the lumen must be made substantially greater than the opening at the window. For example, in the Kolin patent, the window is made somewhat less than 120.degree.; and the coil subtends about 240.degree..
Kolin states (see U.S. Pat. No. 3,309,924) that, in the shutter types using two coils, each coil subtends approximately 60.degree.. For example, in Hickman U.S. Pat. No. 3,580,071, the magnetic influences of two coils subtend approximately 180.degree..
The sensitivity of the lumen expressed as millivolts of signal output per unit of blood flow is related to the angle subtended by the magnetic coils so that the greater is the angle about the lumen encompassed by the coils the greater is the sensitivity. As a rough approximation, the sensitivity may be taken as proportional to the angle subtended by the coils, all other parameters being the same.
All of the above forms of the prior art devices have the disadvantage that the internal diameter of the lumen is greater than the opening at the window, so that either the blood vessel must be squeezed through the window or the lumen will be greater than the diameter of the blood vessels which will impair the functioning of the flow probe.
All of the forms described above have the disadvantage that it requires the use of both hands to manipulate the unit to open and close it.
Another form of probe with shutter-closed openings is the so-called handle probe in which the lumen is mounted on a handle which extends along a prolongation of a diametric plane of the cylindrical lumen. The shutter is mounted on a slide which may be moved away from the lumen to open the window. As in the previous shutter-type lumens, the shutter is substantially less than 180.degree.; and the coils are all mounted in the portion of the lumen attached to the handle. In this form, the opening is of the same angular order and the magnetic field is of the same nature as is described above for the other forms of shutter lumens.